A Location Update procedure concerns one of the most significant mobility management procedures of the UE (user equipment) inside a mobile network according to the GSM (Global System for Mobile communications)/3GPP (3rd Generation Partnership Project) standards. With this procedure, the mobile network has the knowledge on the location of the UE and subsequently the mobile network can apply the various mobile procedures such as calls, Short Message Services, Supplementary Services, location requests, etc.
Each time a UE is registered in a MSC (Mobile Switching Center) node (i.e. it is registered in a VLR (Visited Location Register) service area of the MSC node), the MSC requests subscription information from a HLR (Home Location Register), and at the same time the MSC informs the HLR about the location of the UE. In the successful location update scenario, HLR downloads to the VLR the subscriber's service profile in form of subscriber data.
If the UE is changing the VLR service area, so the UE is registered in a new/target MSC, the previously mentioned process for registration is repeated in the target MSC. In addition to the previous process, HLR orders cancellation of the location of the subscriber in the previous/source MSC. The cancellation of the location of the subscriber in the source MSC results into deregistration of the subscriber, i.e. removal of subscriber data, from the source MSC. This procedure allows the mobile network to keep the location and subscription of the user unique in the networks, i.e. it keeps only the latest location of the UE. Furthermore, this procedure ensures that a subscriber is always registered in a single MSC only.
With the success of the GSM standards based mobile networks worldwide, roaming of UEs can happen between various mobile networks worldwide. So when roaming in visited networks, or when roaming between visited networks of different countries, it can happen that the source MSC, the target MSC, and the HLR are located in different networks of different countries. In this case the signaling of mobility management procedures may traverse a single or even multiple instances of intermediate networks. The condition and quality of intermediate networks may have an impact on the delivery of signaling messages, or may even cause loss of entire signaling messages.
If the location cancellation procedure as a result of location update procedure in a target MSC service area fails, or is not possible due to intermediate networks, MSC, or HLR problems, this may cause problems for the stability of the networks. So if the cancel location message from HLR to source MSC gets lost, the subscriber will remain registered in the source MSC. This means that the subscription of the subscriber is not unique in the mobile network, resulting to capacity and subscription issues in MSCs. Also if the location update request to a target MSC fails, while the HLR has updated the UE location to the target MSC service area and the UE returns to the source MSC, the loss of the cancel location message from HLR to source MSC may cause wrong location information in the HLR.